Quick answer
Locomotives may idle to maintain air pressure, battery charge, engine temperature or cab conditions, but modern locomotives also use anti-idling technology to shut engines down when conditions allow.
A locomotive supports more than traction
The locomotive supplies compressed air for brakes and uses electrical systems that must remain ready. Cold weather and other operating conditions can also affect whether the main engine can be shut down.
Automatic stop-start
EPA rules require anti-idle controls on applicable new and remanufactured locomotives. Automatic systems can shut down the engine and restart it when conditions such as brake-air pressure, battery charge or temperature require it.
Why you may still hear one running
An idle-control system can have legitimate override conditions. A running locomotive is therefore not necessarily being left on without a reason.
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